Heart

• The heart is divided into four chambers, two atria (right and left) and two ventricles (right and left).
• Blood maintains body temp, pH levels, and adequate fluid volume
• Blood is composed of plasma, erythrocytes, leukocytes, and thrombocytes
• Hematocrit is the percentage of RBC out of a total blood volume
• Heart sounds:

  • First sound (LUB): AV valves close; signifies beginning of ventricular systole (contraction)
  • Second sound (DUB): Semilunar valves close at beginning of ventricular diastole (relaxation)
• The Cardiac Cycle:

  • Period from one heartbeat to the beginning of the next
  • Alternating periods of contraction (systole) and relaxation (diastole)
  • Atria contract as a pair first as ventricles are relaxed (filling)
  • Ventricles contract as a pair as atria are relaxed (filling)
  • Typical cardiac cycle lasts around 800 msec; all four chambers are relaxed at the beginning
• Cardiac Cycle: Atrial systole / Ventricular diastole:

  • At start of atrial systole, ventricles are already 70% full from passive filling during the previous cardiac cycle
  • Heart blood pressure is low as blood enters atria & flows into ventricles
  • Atrial systole "tops off" ventricles with maximal end-diastolic volume (EDV)
• Cardiac Cycle: Ventricular systole:

  • Atria relax
  • Rising ventricular pressure closes AV valves
  • Ventricular ejection (systole) opens semilunar valves
• Cardiac Cycle: Ventricular Diastole:

  • Ventricles relax
  • Backflow of blood in aorta & pulmonary trunk closes semilunar valves
  • Atrial pressures force AV valves open for blood to flow through relaxed atria & ventricles
• Cardiac Output (CO):

  • Amount of blood pumped by each ventricle in one minute
  • CO = heart rate (HR) x stroke volume (SV)
  • HR = number of heart beats per minute
  • SV = amount of blood pumped by a ventricle with each beat
• Regulation of Stroke Volume (SV):

  • SV = end diastolic volume (EDV) - end systolic volume (ESV)
  • EDV is the amount of blood collected in a ventricle during diastole (preload)
  • ESV is the amount of blood remaining in a ventricle after contraction
• Factors affecting stroke volume:

  • Influences on EDV: venous return, filling time, preload
  • Starling's Law: the greater the stretch of cardiac muscle, the greater the contraction
  • Influences on ESV: contractility, afterload
• Altered IONS Influence Contractility/ESV/Stroke Volume:

  • Hyperkalemia: increased extracellular K+ can lead to weakened contractions
  • Hypocalcemia: decreased extracellular Ca2+ can cause weak contractions or cessation of contractions
• CAD (Coronary Artery Disease):

  • Atherosclerosis in coronary vessels leads to reduced blood flow to the heart
  • Plaques may be visible in clinical scans like digital subtraction angiography (DSA)
• Cardiac Disorders:

  • CAD (coronary artery disease) early sign: angina pectoris
  • Medications for CAD include vasodilators (Nitroglycerin), calcium channel blockers, and blood thinners like Coumadin and Aspirin
• Surgical Treatment for CAD:

  • Balloon angioplasty with stent
  • Coronary Artery Bypass Surgery using part of great saphenous vein &/or internal mammary artery
• Myocardial Infarction:

  • Heart attack commonly due to CAD
• (Congestive) Heart Failure caused by coronary artery blockages, multiple myocardial infarcts, or persistent high blood pressure
• Myocarditis:

  • Viral infection is the most common cause
• COVID-19 and Myocarditis:

  • 16 times greater risk of myocarditis in patients who had Covid-19 compared to those who did not